1. Technical Field
The present disclosure relates to a process for manufacturing a micromechanical structure having a buried area provided with a filter. In one embodiment, the micromechanical structure is formed in a body made of semiconductor material, in particular silicon, using micromachining techniques and finds advantageous use, for example, for microfluidic applications.
2. Description of the Related Art
As is known, in the semiconductor industry micromachining techniques enable manufacturing of micromechanical or MEMS (microelectromechanical systems) structures, which have a wide range of applications.
In particular, in the microfluidic field, it is common to manufacture structures including buried areas (for example channels, chambers, or cavities) within a silicon substrate (or other semiconductor material body), which are for example used for enabling passage of fluids, such as drugs that are to be administered to a patient or inks used for printing operations, or else for enabling even complex chemical reactions, such as reactions of multiplication of DNA sequences through the PCR (polymerase chain reaction) process.
In general, the manufactured buried areas enable an environment to be obtained that is substantially separate and distinct from the external environment (and possibly communicates with the external environment by means of one or more access ducts), in which chemical reactions, exchanges or flows of fluids take place in a controlled manner.
Known processes for manufacturing of microfluidic structures provided with buried areas generally envisage the use of techniques of processing from the back of the silicon substrate or wafer (or other body of semiconductor material). These processes envisage chemical etching of the back of the substrate to define a first wall, for example a top wall, of the area that is to be buried (which defines, for example, a channel or a cavity), and next bonding of the substrate with a different structural body (for example, a plate of glass or an adhesive layer), such as to close the buried area defining a corresponding second wall thereof, for example a bottom wall. These operations can be carried out, in part, during the processes for assembling the package or “molding” of the microfluidic structures (operations known as a whole as “back-end” operations).
In addition, ducts for access to the buried area can be provided with purposely designed etches from the front of the substrate (techniques of silicon processing known as a whole as “front-end” techniques).
These manufacturing processes have, however, certain drawbacks that do not enable full exploitation of the characteristics thereof, amongst which:                the techniques of back processing, and the subsequent operations of bonding between different bodies, entail generally the generation of undesirable particles and contaminants, which can jeopardize operation of the final microfluidic device; and        the back-end and bonding operations are in general complex, costly, and generally require long processing times.        
In particular, the problem of contamination from external particles, which is not due only to the aforesaid manufacturing operations and to the residue of processing, but also to the presence for other reasons of external particles that can interact with the buried area, is particularly felt, in so far as this contamination can jeopardize the performance of the microfluidic devices or proper execution of the operations of delivery or passage of the fluids associated thereto.
For this reason, the use of more or less complex filters has been proposed, which are designed to be coupled to at least one access duct (inlet or outlet duct) in fluid communication with the buried cavity. In particular, given that these filters have pores of micrometric or sub-micrometric dimensions, they enable filtering and subsequent removal of possible impurities during flow of the liquids.
For example, U.S. Pat. No. 5,753,014 discloses the formation of a membrane filter by means of chemical etching of a silicon membrane having a thickness of a few microns. In particular, the membrane is obtained by means of chemical etching from the back of a silicon substrate, and a desired pattern of micrometric or sub-micrometric pores is subsequently defined through the membrane with photolithographic techniques. The membrane filter thus obtained can be coupled, as an external element, to structures with buried areas or ducts with a diameter (size) of from a few microns up to hundreds of microns or even millimeters, for operations of filtering of the incoming/outgoing fluids.
This solution is not, however, optimized from the standpoint of simplicity and economy of the manufacturing process, for example because it requires complex steps of coupling of the filter to structures formed separately, it requires to comply with specifications of mechanical and/or optical alignment, and moreover it does not enable in any case reduction of the contaminations in the manufacturing process of the buried areas.
Solutions are also known in which the filtering element is defined by bonding two bodies so as to define vertical pillars arranged according to desired lattices in a direction transverse to the direction of flow of the fluids. These solutions suffer from further disadvantages, amongst which: the process of bonding of the various bodies suffers from misalignment inaccuracy, the value of which (for example +/−20 μm) may not enable formation of sufficiently small filtering pores (for example of a diameter of 0.5 μm); during the bonding operation, the distance between the vertical pillars and the body facing them may not be sufficiently repeatable for enabling an adequate filtering action; and furthermore, in order to obtain adequate flows of fluids, the buried channels have, for example, a height of some tens of microns (for example, a height of 50 μm), with the consequent fragility of the aforesaid pillars that extend vertically from the bottom wall to the top wall of the channels (failure of the pillars can jeopardize the filtering action and can itself cause impurities in the flow of the fluids).